1. Field of the Invention
The present invention relates generally to handling nuclear fuel pellets and, more particularly, is concerned with a nuclear fuel pellet escalator for transferring pellets from a rotary pellet press into a sintering boat.
2. Description of the Prior Art
An operational step in the nuclear fuel fabrication process is the loading of green nuclear fuel pellets, which have been ejected from the pellet press, into sintering containers, or "boats", in preparation for high-temperature firing of the pellets in a sintering furnace. This operation requires careful handling of the pellets, because the pellets at this time are fragile and susceptible to damage.
One approach in the prior art to loading nuclear fuel pellets into a sintering boat is by gravity discharge from the pellet press down a chute into the boat. This approach has the disadvantage of allowing significant amount of pellet-to-pellet impacts, providing considerable potential for pellet damage. Another approach in the prior art is to employ mechanical blades or jaws to pick up a single row of nuclear fuel pellets at a time and transfer that row into the sintering boat. Still another prior art approach is to utilize a vacuum transfer head which lowers onto an array of nuclear fuel pellets, applies a vacuum to lift the pellets, and transfers the array to load a complete layer of pellets into the sintering boat at one time.
Two other approaches for loading nuclear fuel pellets into sintering boats from a pellet press are disclosed in U.S. Pat. No. 4,332,120 to Haynes et al and U.S. Pat. No. 4,566,835 to Raymond et al assigned to the assignee of the present invention. In the Haynes et al patent, a loading mechanism is disclosed which includes a rotating table on the pellet press that carries nuclear pellets from the press, a stationary ejector arm which cams pellets from the rotating table, a rotating turntable located adjacent the rotating press table which receives the pellets cammed therefrom, and another ejector arm or camming bar that cams pellets off the rotating turntable and over an opening through which the pellets can fall into a vertical chute located above a sintering boat. The vertical chute has a plurality of oppositely downwardly-angled resiliently-covered plates defining a zig-zag path downward through the chute for directing pellets downwardly by gravity from the rotating turntable at the higher elevation to the sintering boat at the lower elevation. The zig-zag configuration of the path serves to limit the accumulation of momentum by the pellets as they fall to the sintering boat. A platform supporting the boat is spring-biased against the accumulated weight of the pellets and boat so as to continuously lower as the weight of the pellets deposited in the boat increases.
In the Raymond et al patent, a loading system is disclosed which includes a second rotating transfer turntable that receives pellets ejected from a first rotating turntable of a pellet press by a first stationary wiper arm, a transfer conveyor which receives pellets guided off the second transfer turntable by a second stationary wiper arm, and a rotatable drum which receives pellets in a row thereof from the transfer conveyor and transfers them to one of a pair of sintering boats positioned on sliding tables on either side of the drum. The transfer drum is rotatable about a horizontal axis and has three longitudinally extending channels being circumferentially spaced from one another 120 degrees. Each channel is sized to receive a row of standing pellets when at an upward vertical loading position. The drum can be rotated and counterrotated to change the position of a given channel from the upward vertical loading position to a below-horizontal unloading position and back around to the upward loading position. Each channel has a bottom bar which can be moved by an actuator to eject the row of pellets when the channel is at the below-horizontal unloading position and aligned with an incline member leading to one of the boats.
The above-described approaches of Haynes et al and Raymond et al appear to constitute steps in the right direction. However, these approaches still fall short of providing the degree of control over handling of green nuclear fuel pellets that is desired. Consequently, further improvements are still needed in the loading of green nuclear fuel pellets.